Atmospheric Pollution

Chapter 17

Atmospheric Pollution M.L. Brusseau, A.D. Matthias, A.C. Comrie and S.A. Musil

Smelter-produced air pollutants trapped by an inversion layer. Photo courtesy J.F. Artiola.

17.1 AIR POLLUTION CONCEPTS In this chapter, we discuss air pollutants, including their sources and effects on human activity, as well as their transport to, and fate in, the atmosphere. We also describe the role of air pollution in such major environmental issues as global climate change and stratospheric ozone depletion that are covered in Chapter 25. An air pollutant is any gas or particulate that, at high enough concentration, may be harmful to life, the environment, and/or property. A pollutant may originate from natural or anthropogenic sources, or both. Pollutants occur throughout much of the troposphere (see Chapter 4); however, pollution close to the earth’s surface within the boundary layer is of most concern because of the relatively high concentrations resulting from sources at the surface. Atmospheric pollutant concentrations depend mainly on the total mass of pollution emitted into the atmosphere, together with the atmospheric conditions that affect its transport and fate. Obviously, air pollution has many and varied sources, including cars, smokestacks, and other industrial inputs into the atmosphere as well as wind erosion of soil. Large emissions from both anthropogenic and natural sources over long periods enhance concentrations, as do the chemical and physical properties of these pollutants. For example, when nitrogen oxides and hydrocarbons in car exhaust are emitted into warm, sunlit air,

Environmental and Pollution Science. https://doi.org/10.1016/B978-0-12-814719-1.00017-3 Copyright © 2019 Elsevier Inc. All rights reserved.

they readily form ozone molecules (O3). Similarly, the solubility of a pollutant affects how efficiently it is removed by rainfall. Atmospheric conditions have a major effect upon pollutants once these pollutants are emitted into (e.g., nitrogen oxides from car exhaust) or formed within (e.g., O3) the atmosphere. Pollution dispersal is controlled by atmospheric motion, which is affected by wind, stability, and the vertical temperature variation within the boundary layer. Stability, in turn, influences both air turbulence and the depth at which mixing of polluted air takes place. Wind determines the horizontal movement of pollution in the atmosphere. Pollution emitted from a point source, such as a smokestack, is generally dispersed downwind in the form of a plume (see Chapter 7). Wind speed establishes the rate at which the plume contents are transported. Strong winds flowing over rough land surfaces enhance mixing of air by producing shear stress (mechanical mixing) much like that created when an electric fan circulates air in a room. Also, wind direction establishes the path followed by the pollution. Once present in the atmospheric boundary layer, a pollutant may undergo a series of complex transformations leading to new pollutants, such as O3. Also, the removal of pollutants from air by rain and snow, by gravity, or by surface deposition is influenced by boundary-layer conditions. These removal processes, in turn, are also affected by the type and roughness of the underlying ground surface.

293

294

PART

II Environmental Pollution

Even when emissions are relatively constant, pollutant concentrations can quickly change, owing to variations in atmospheric conditions. When atmospheric conditions are stable, relatively low emissions can cause a buildup of pollution to hazardous levels. This situation can occur during radiation inversions at night (see Section 4.2.2). In contrast, unstable conditions may effectively dilute pollution to relatively “safe” concentrations despite a fairly high rate of emissions. Air pollution, which is of major public concern (Chapter 26), is currently the object of extensive scientific research. Its effects on life, including human health, productivity, and property, are not yet fully understood, even though exposure to high levels of pollution is a daily experience for many people. The cost of such pollution, whether expressed in terms of direct biological consequences or in terms of economic impact, is enormous. Worldwide, urban air pollution affects nearly a billion people, exposing them to possible health hazards. In the United States alone, billions of dollars are spent annually to prevent, control, and clean up air pollution; other developed nations are incurring similar costs. The United Nations considers air pollution to be a major global problem. Most commonly, air pollution poses a health risk that can and does harm life. It harms the human respiratory and pulmonary systems. Emphysema, asthma, and other respiratory illnesses may result from or be aggravated by chronic exposure to certain pollutants, such as O3 or particulates. Research conducted in Southern California indicates that breathing polluted air slows lung development in children as much as having a parent who smokes tobacco. Teenagers who are chronically exposed to polluted air are five times as likely to have reduced lung function as are teens who breathe clean air. Children and adults with chronic exposure to elevated ozone levels are more likely to develop asthma than individuals in cleaner air (see Information Box 17.1). Breathing polluted air can also thicken human artery walls, which is an important risk factor for heart failure and strokes. In 1991, researchers at the U.S. Environmental Protection Agency (EPA) concluded that about 60,000 U.S. residents die each year due to heart attacks and respiratory illness caused by breathing particulates (dust) at concentrations within the federal PM10 (see Section 17.2.1.3) air quality standard. Air pollution also poses an ecological risk. Vegetation can be harmed by uptake of pollutants through the leaf stomata or by deposition of pollutants on the leaf surfaces. Sufficiently high concentrations of sulfur dioxide or ozone, for example, may cause leaf lesions in susceptible plants. Chronic exposure to relatively low levels of pollution can harm plants by reducing their resistance to disease and insect predators. Crop yields can be lowered by air pollution. The presence of certain air pollutants, such as ozone

INFORMATION BOX 17.1 Asthma and Air Pollution There is much concern about the possible adverse health effects of air pollution on respiratory health, especially of children within inner-city areas. The U.S. EPA is currently conducting studies of the potential role that several air pollutants may have in inducing and/or exacerbating asthma or asthma symptoms. The incidence of asthma has increased dramatically in the United States during recent decades, the causes of which are not well understood. Increased air pollution in the form of complex mixtures of particulate matter, metals, and tobacco smoke is thought to play an important role in the increase. Indoor air quality is a particular concern (see Chapter 18). Another class of hazardous chemicals that may have an important role is the carbonyl compounds, such as aldehydes and ketones. They can originate from industrial sources and from the burning of diesel fuel. They also form within the troposphere during complex photochemical processes involving organic compounds.

(formed from nitrogen oxides and hydrocarbons), enhances the earth’s natural greenhouse effect within the troposphere. This enhancement warms the earth and may change rainfall patterns, which could markedly alter the distribution of life on earth (see Chapter 25). Air pollution can also damage property. It can erode the exterior surfaces of buildings, particularly those constructed of limestone materials that react with acids in precipitation (see Chapter 4). Further evidence of the deleterious effects of air pollution on property can be seen in the damaged paint finishes on cars regularly parked downwind from ore smelters.

17.2 SOURCES, TYPES, AND EFFECTS OF AIR POLLUTION Air pollution is not a new problem. Lead in Swedish lake sediments indicates that air pollution produced from lead mining and silver production in ancient Greece and Rome affected air quality throughout Europe. Early written accounts of air pollution refer mainly to smoke from burning wood and coal. For example, in the 13th century, King Edward I of England prohibited the use of sea coal, the burning of which produced large amounts of soot and sulfur dioxide (SO2) in the atmosphere over London. The Industrial Revolution increased pollution so markedly that air quality deteriorated significantly in Europe and North America. By the mid-19th century, many cities in the United States and Europe were experiencing the consequences of air pollution. By the early 20th century, the term “smog” was coined to describe the adverse combination of smoke and fog in London. In Los Angeles, photochemical

Atmospheric Pollution Chapter

17

295

INFORMATION BOX 17.2 Air Pollution in China China is the second largest energy user in the world, due in part to its rapid industrialization. In 2002, about two-thirds of China’s energy came from burning coal, which tends to release large amounts of soot and SO2, depending on the quality of the coal. This is particularly troublesome, since much of the coal emissions are from older industrial plants with poor pollution controls, as well as from domestic use (individuals using coal to cook and heat homes). The high levels of domestic use create numerous small point sources, low to the ground, that are difficult to control or abate (see Section 17.3 for more information on air pollution dispersion). As a result, some cities are establishing “coal-free zones” and are attempting to increase the use of cleaner burning natural gas. The World Health Organization noted in a 1998 report that seven of the ten most polluted cities in the world are in China. The government has begun to implement pollution reduction programs, but they face enormous problems, including acid rain, which is estimated to fall on about 30% of China’s territory. The government is trying to improve enforcement of existing laws, as well as instituting fines on polluters, designing systems for emissions trading (already in use in North America and Europe), and focusing on new technology to reduce energy use and pollution.

smog alerts became common by the mid-1940s. The first major air pollution disaster in the United States occurred in 1948, when approximately 20 lives were lost as a result of industrial pollutants trapped in very stable air over Donora, Pennsylvania, in the Monongahela River Valley. During one week in December 1952, stagnant air and coal burning caused severe smog conditions in London that ultimately took the lives of nearly 12,000 people over a threemonth period. Virtually all metropolitan areas are affected by air pollution, especially those situated in valleys surrounded by mountains (e.g., Mexico City) or along coastal mountain ranges (e.g., Los Angeles). Air pollution has recently become a critical issue for cities in countries experiencing rapid economic development, such as China and India (see Information Box 17.2 and Fig. 17.1 for an example from China). But even unpopulated areas far from cities may be affected by long-range transport of pollution, either from urban areas or from such rural sources as ore smelters or coal-burning power plants. For example, pollution from a coal-burning power plant in northern Arizona reduces visibility in Grand Canyon National Park, located 400 km west of the plant. Most of the air we breathe is elemental oxygen (O2) and nitrogen (N2) (see Chapter 4). About 1% is composed of naturally occurring trace constituents such as carbon dioxide (CO2) and water vapor. A small part of this 1%

FIG. 17.1 A smokestack contributing to smog in Tianjin, China. (Photo courtesy J. Walworth.)

may, however, be air pollutants, including gases and particulate matter suspended as aerosols. Anthropogenic air pollution enters the atmosphere from both fixed and mobile sources. Fixed sources include factories, electrical power plants, ore smelters, and farms, while mobile sources include all forms of transportation that burn fossil fuels. Mobile sources account for 56% of the pollutants emitted to the atmosphere in the United States (Fig. 17.2A). Fuel combustion from stationary sources accounts for nearly 15%, and industrial processes account for about 7% of emissions in the United States. Natural sources of air pollution include winds eroding dust from cultivated farm fields, smoke from forest fires (Fig. 17.3), and volcanic ash that is emitted into the troposphere and stratosphere (see Information Box 4.1). There are many types of air pollutants. Some gases, such as CO2, produced by burning fossil fuels, are central to the issue of global climate change but are also essential to plant life. Many pollutants, such as dust particles, exist naturally in the atmosphere and become hazardous only when their concentrations exceed air-quality standards set by such regulatory agencies as the U.S. EPA. The EPA classifies air pollutants according to two broad categories: primary and secondary air pollutants.

296

PART

II Environmental Pollution

l l l l l l

carbon monoxide hydrocarbons particulate matter sulfur dioxide nitrogen oxides lead

17.2.1.1 Carbon Monoxide

FIG. 17.2 (A) Source contributions of primary air pollutants in the United States in 2002 and (B) the percentage of total primary air pollutants by type. (Data: U.S. Environmental Protection Agency.)

Carbon monoxide (CO), which is the major pollutant in urban air, is a product of incomplete combustion of fossil fuels. Carbon monoxide has relatively few natural sources. It is a part of cigarette smoke, but the internal combustion engine is the major source, with about 50% of all CO emissions in the United States originating from cars and trucks. Emissions, therefore, are highest along heavily traveled highways and streets (Fig. 17.4). Of the EPA-designated primary pollutants in the United States, CO emissions currently contribute about 60% of the total emissions (see Fig. 17.2B). Fortunately, CO concentrations are decreasing in the United States, because newer cars have higher fuel efficiencies. Carbon monoxide is highly poisonous to most animals. The EPA standards currently limit human exposure to a 24-h average of 9 nL L–1 or a 1-h average of 35 nL L–1. (Note: The alternative unit of parts per billion (ppb) is also commonly used. See Chapter 4 for more information on units for describing gas concentrations.) When inhaled, CO reduces the ability of blood hemoglobin to attach oxygen. Although relatively stable, it is short-lived in the atmosphere because it is quickly oxidized to CO2 by reaction with hydroxide radicals. Some atmospheric CO may be removed by soil microbes. In order to increase the oxidation of CO to CO2 during fuel combustion, some cities require the use of oxygenated gasoline containing ethanol or other additives during winter months.

FIG. 17.3 A natural source of atmospheric pollution. The “Aspen Fire” burned for weeks in the Catalina Mountains near Tucson, Arizona, releasing large amounts of particulates and other air pollutants. (Photo courtesy J.F. Artiola.)

17.2.1

Primary Pollutants

Primary air pollutants enter the atmosphere directly from various sources. The EPA designates six types of primary air pollutants for regulatory purposes:

FIG. 17.4 Emissions of CO and nitrogen oxides are highest along heavily traveled highways and streets. (Photo courtesy S.A Musil.)

Atmospheric Pollution Chapter

17.2.1.2 Hydrocarbons Hydrocarbons (HCs), or volatile organic compounds (VOCs), are compounds composed of hydrogen and carbon. Methane (CH4), the most abundant hydrocarbon in the atmosphere, is an active greenhouse gas. Volatile organics include the nonmethane hydrocarbons (NMHCs), such as benzene, and their derivatives, such as formaldehyde. Some of these compounds (e.g.,benzene) are carcinogenic, and some relatively reactive HCs contribute to ozone production in photochemical smog. Hydrocarbons are produced naturally from decomposition of organic matter and by certain types of plants (e.g., pine trees, creosote bushes). In fact, HCs emitted from vegetation may be a major factor in smog formation in some cities, particularly those near forested areas of the southeastern United States. A large proportion of HCs and NMHCs are generated by human activity. Some NMHCs, including formaldehyde, are readily emitted from indoor sources, such as newly manufactured carpeting. Hydrocarbons are also emitted into the atmosphere by fossil fuel combustion and by evaporation of gasoline during fueling of cars. To mitigate

17

297

this latter source, some municipalities require that servicestation gasoline pumps be fitted with a special trap to collect HC vapors emitted during fueling of vehicles. Because transportation is the primary source of HCs, concentrations tend to be highest near heavily traveled roadways.

17.2.1.3 Particulate Matter The category of particulate matter comprises solid particles or liquid droplets (aerosols) small enough to remain suspended in air. Such particles have no general chemical composition and may, in fact, be very complex. Examples include soot, smoke, dust, asbestos fibers, and pesticides, as well as some metals (including Hg, Fe, Cu, and Pb). We can characterize particulate matter by size. Particles whose diameters are 10 μm or larger generally settle out of the atmosphere in less than a day, whereas particles whose diameters are 1 μm or less can remain suspended in air for weeks. Smaller particulate matter, whose particles are 10 μm or less, have come to be known as PM10. The very small particles with diameters 2.5 μm or less are known as PM2.5 (see Information Box 17.3 and Chapter 11).

INFORMATION BOX 17.3 Aerosols and Visibility (See also Chapter 11) Aerosols are solid or liquid particles suspended in a gas. In relation to environmental pollution, our major concern is with microscopic aerosol particles produced primarily from combustion or windblown dust, or secondarily from gas-to-particle conversion. Aerosols in the atmosphere typically fall into two distributions by mass or size, with coarse particles in the range 2.5–10μm and fine particles from about 0.1 to 2.5 μm. The coarse fraction is usually composed of soil dust including minerals and organic particles. Fine particulate matter <2.5 μm (PM2.5) comprises a range of combustion products such as elemental carbon, sulfates, and nitrates. Aerosols are important constituents of acid rain, and they can also alter the atmospheric radiation balance. Because their small size allows them to penetrate deep into the lungs, aerosols have important health effects, and therefore the EPA has established health-based standards for PM2.5 and PM10. Visibility impairment, especially at the regional scale, is typically caused by fine aerosols, especially <1 μm, as this size range tends to remain suspended longest in the atmosphere (coarser particles may settle out and ultrafine particles can be removed as condensation nuclei in rain-out) (see Fig. 17.5). These fine particles also scatter relatively large amounts of light, leading to greater haziness and decreased visibility. Visibility impairment has become a problem in many areas, not only in the eastern United States, but also in relatively remote places such as the Grand Canyon. The Western Regional Air Partnership is a coordinated effort by the Western Governors’ Association and the National Tribal Environmental Council to develop data, tools, and policies needed by states and tribes to improve visibility in parks and wilderness areas across the western United States.

FIG. 17.5 Photos taken of Mount Trumbull in the Grand Canyon National Park on a “clear” (A) day and a “hazy” (B) day at noon. The figure to the right clearly shows the impairment of visibility due to aerosols, some of which originate in California urban areas. The National Park Service has a visibilitymonitoring program that systematically collects photos at numerous parks across the United States. (Photos courtesy National Park Service.)

298

PART

II Environmental Pollution

The effects of particulates in the air are various. Some particulates, especially those containing sulfur compounds, are emitted by volcanoes. These particulates can reach the stratosphere, where they may significantly alter the radiation and thermal budgets of the atmosphere and thus produce cooler temperatures at the earth’s surface (see Chapter 4). Tropospheric particulates may cause or exacerbate human respiratory illnesses. Especially harmful to the human respiratory system is the fraction of mid-sized particles, PM10 and PM2.5. In large cities, particulates also reduce visibility. In the United States in 2002, about 62% of particulates came from roads and transportation, with another 26% contributed by agriculture, forestry, and fires (Fig. 17.6). Construction is now considered to make a large contribution to PM10 levels.

17.2.1.4 Sulfur dioxide About 90% of sulfur dioxide (SO2) emissions come from burning sulfur-containing fossil fuels, such as coal, which may contain up to 6% sulfur. Ore smelters and oil refineries also emit significant amounts of SO2. At relatively high concentrations, SO2 causes severe respiratory problems. Sulfur dioxide is also a source of acid rain, which is produced when SO2 combines with water droplets to form sulfuric acid (H2SO4). At sufficiently high concentrations, SO2 exposure is harmful to susceptible plant tissue. Sulfur dioxide and other tropospheric aerosols containing sulfur are believed to affect the radiation balance of the atmosphere, which may cause cooling in certain regions. See Chapter 4 for information about the contribution of SO2 to acid rain.

17.2.1.5 Nitrogen Oxides (NOx) Nitrogen oxides (NOx stands for an indeterminate mixture of NO and NO2) are formed mainly from N2 and O2 during high-temperature combustion of fuel in cars. Catalytic converters are used to reduce emissions. Nevertheless, NO

causes a reddish-brown haze in city air that contributes to heart and lung problems and may be carcinogenic. Nitrogen oxides also contribute to acid rain because they combine with water to produce nitric acid (HNO3) and other acids. Natural sources of nitrogen oxides include those produced during the metabolism of certain soil bacteria.

17.2.1.6 Lead Lead is highly toxic, and its effects on humans have been recognized since the Roman Empire era. Lead can produce chronic impairment of the formation of blood and it affects infant neurological development (see Chapter 26). Concentrations of lead in the environment in the United States are no longer as high as they were prior to the introduction of nonleaded gasoline in the 1970s, but it is still a concern in many localities. Lead from human sources may be present in soil and it is often found in particulate matter in older urban environments. Lead-based paints continue to be a source of concern in situations where children are exposed to paints that have peeled from building surfaces. Leadbased paint was banned in the United States in 1978. Homes built before 1978 may have lead-based paint that can be a health hazard when sanded, chipped, or removed.

17.2.2

Secondary Pollutants

Secondary air pollutants are formed during chemical reactions between primary air pollutants and other atmospheric constituents, such as water vapor. Generally, these reactions must occur in sunlight; thus they ultimately produce photochemical smog (see Information Box 17.4). Photochemical smog is most common in the urban areas where solar radiation is very intense. A simplified set of some of the reactions involved in photochemical smog formation is given as follows: N2 + O2 ! 2NO 2NO + O2 ! 2NO2 NO2 + hv ! NO + O O + O2 ! O3 NO + O3 ! NO2 + O2 HC + NO + O2 ! NO2 + PAN

ðinside an engineÞ ðin the atmosphereÞ ðin the atmosphereÞ ðin the atmosphereÞ ðin the atmosphereÞ ðin the atmosphereÞ (17.1)

As indicated by the reactions in Eq. (17.1), photochemical smog is composed mainly of O3, peroxyacetyl nitrate (PAN), and other oxidants. Ozone formation is closely tied to weather conditions. Favorable conditions for O3 formation include: l

FIG. 17.6 Agricultural crops and livestock contributed about 19% of particulate matter in 2002 in the United States. (Photo: College of Agriculture and Life Sciences, The University of Arizona.)

l l l

air temperatures exceeding 32°C low winds intense radiation low precipitation

Atmospheric Pollution Chapter

INFORMATION BOX 17.4 Photochemical Smog in Los Angeles Photochemical smog can be severe in the Los Angeles basin of the California coast (see Fig. 17.7). Commuting in Los Angeles requires many cars, which produce high emissions of NOx and hydrocarbons. At certain times of the year, particularly spring and fall, weather conditions in this area are dominated by subtropical high pressure with clear, calm air conditions that exacerbate air stagnation. The factors influencing smog formation in the Los Angeles basin can be summarized as follows: l Numerous sources of primary pollutants. l Inversions that inhibit turbulent mixing of air. l Few clouds, which result in higher UV intensity. l Light winds that are unable to disperse pollutants. l Complex coastal mountain terrain that slows pollutant dispersal.

FIG. 17.7 Hydrocarbons interact with nitrogen oxides under the influence of ultraviolet light, resulting in photochemical smog. In urban centers such as Los Angeles, pictured here, atmospheric pollutants can concentrate and pose severe health hazards. (Photo: U.S. Environmental Protection Agency.)

Unfortunately, many major U.S. cities exceed the federal air-quality standard for O3 (an average O3 concentration >80 nL L–1 for 1 h 1 day per year averaged over a 3-year period). As the reactions indicate, HCs are necessary for ozone buildup in the atmosphere. In the absence of HCs, solar UV breaks down the NO2 into NO and O. Next, the O atom combines with O2 to form O3, which then combines with the NO to reform NO2 and O2. Ozone would not accumulate in the atmosphere if it was not for the fact that HCs disrupt the reaction cycle by reacting with NO to form PAN + NO2. Hydrocarbons from car emissions and other sources, therefore, play an important part in O3 formation in urban environments. However, not all O3 in the lower atmosphere results from human activities; natural sources include lightning and the diffusion of some O3 downward from the stratosphere.

17

299

In most western U.S. cities, photochemical smog is often referred to as brown cloud (O3 + PAN + NOx). Industrial eastern and midwestern U.S. cities also have photochemical smog, but they generally receive less intense sunlight than western cities; thus smog in those cities is sometimes referred to as gray air because of particles (especially PM2.5) and SO2 emanating from burning fossil fuel. Ozone may be either hazardous or beneficial, depending largely on where it is. For example, it is hazardous as an oxidant in smog (ground-level ozone), but in the O3 layer in the stratosphere, it is beneficial because it absorbs UV radiation. Smog ozone reduces the normal functioning of lungs because it inflames the cells that line the respiratory tract. Other health effects include increased incidence of asthma attacks, increased risk of infection, and reduced heart and circulatory functions. Smog O3 can also damage plant life. In vegetation, the main damage occurs in foliage, with smaller effects on growth and yield. In the United States, it has been implicated in the loss of conifer trees near Los Angeles and is suspected of doing similar damage to trees in the Appalachian Mountains. Some plant species are also very susceptible to PAN in smog, and this is known to affect plants in the Los Angeles area. Ozone and NOx pollution in the troposphere is not limited to urban areas. In the mid-1990s, the EPA reported that O3 and NOx concentrations were increasing in rural areas in the southeastern and midwestern United States (see Information Box 17.5). Most of this increase is probably attributable to upwind urban sources; however, in some rural areas, soil bacteria may be a greater NOx source than is fossil fuel combustion. In fact, some estimates indicate that soil bacteria may emit as much as 40% of the total amount of NOx emitted into the atmosphere. This percentage is very uncertain, since data on measurements of NOx fluxes from soils are scanty. Soil NOx fluxes may also be highly spatially and temporally variable. Tropospheric NOx strongly controls the concentrations of oxidants such as OH and O3, which may affect the health of about one-quarter of the U.S. population. As in urban areas, O3 in the rural atmosphere is controlled by reactions involving NOx, HCs, OH, and other tropospheric species. The study of the production and destruction processes of O3 in the rural troposphere is currently an area of active research by the EPA and other government agencies. Many questions remain unanswered concerning the reasons why O3 concentrations continue to be high in both urban and rural areas despite recent efforts to curb emissions of the O3 precursors, NOx, and HCs.

17.2.3

Toxic and Hazardous Air Pollutants

In addition to primary and secondary pollutants, the EPA has identified 188 chemicals (or classes of chemicals) that

300

PART

II Environmental Pollution

INFORMATION BOX 17.5 Regional Ozone Transport Ground-level ozone (“smog”) problems are not just limited to cities. Several times each summer, the central and eastern United Statescomes under the influence of stagnating high-pressure weather systems. Sunshine, high temperatures, and stagnant nonmixing conditions exacerbate the buildup of ozone precursor pollutants and the production of ozone over areas with high pollutant emissions, such as the industrialized Midwest. The slow flow of the polluted air mass traverses multiple states between the Midwest and the Atlantic Coast and can raise concentrations of ozone to unhealthy levels in rural as well as urban areas (Fig. 17.8).

FIG. 17.8 Wind can move polluted air across long distances. As a result, rural areas may experience polluted air that originated in metropolitan areas. The black arrows indicate transport winds, where longer arrows indicate faster movement. The color contours indicate ozone concentration. (Source: Ozone Transport Assessment Group, 1997.)

are considered to be hazardous air pollutants (HAPs) or urban air toxics (UATs). Many of these are volatile organic chemicals, such as benzene found in gasoline and used as a solvent, and trichloroethene, which is used as a solvent/ degreaser (see Chapter 12). Mercury is an example of a hazardous inorganic compound (see Information Box 17.6).

17.2.4

Pollutants With Radiative Effects

Some air pollutants greatly influence the interactions between radiation of various wavelengths and the atmosphere. Some radiatively active pollutants contribute strongly to the natural greenhouse effect, while others impact the amount of ozone present within the stratosphere. This section describes these pollutants and their radiative effects.

17.2.4.1 Greenhouse Gases Carbon dioxide is sometimes not considered to be an air pollutant because it is not hazardous to human health at ambient

atmospheric concentrations; moreover, it is essential for carbon fixation by plants. It is, however, an important greenhouse gas and a major by-product of fossil-fuel burning, which steadily increases the atmospheric concentration of carbon dioxide. It therefore plays a central role in global climate change. This increase is discussed in detail in Chapter 25. Carbon dioxide is by far the most abundant and important atmospheric trace gas contributing to the natural greenhouse effect (see also Chapter 4). It is released to the atmosphere by various processes including deforestation and land clearing, fossil-fuel combustion, and respiration from living organisms. Carbon dioxide is readily absorbed by water, with warm water absorbing more than cold water, so it is removed from the atmosphere by the oceans and other bodies of water. Photosynthesis by land and water plants (phytoplankton) also removes significant amounts of CO2 from the atmosphere. Removal by plants is particularly apparent during the summer, when average CO2 concentrations decrease. In addition, large amounts of CO2

Atmospheric Pollution Chapter

INFORMATION BOX 17.6 How Does Mercury Get in My Food? More than 40% of all mercury emissions in the United Statesare from power plants burning coal (about 50 tons Hg yr–1 in 2005). The mercury is released as a gas, which eventually is deposited into water or soil. Microbes can then convert inorganic mercury to methyl-mercury in a process called methylation. Small organisms can take up small amounts of methyl-mercury as they feed, storing it in their tissues. As these organisms are eaten by animals higher in the food chain, the methyl-mercury continues to accumulate in body tissues, until they may reach relatively high levels in predators at the top of the food chain, such as swordfish and sharks. The methyl-mercury then becomes part of our food as we consume shellfish or fish from saltwater and freshwater sources. Mercury tends to accumulate in tissue, so toxic levels can build up slowly. High levels of exposure to methyl-mercury can damage the human brain, resulting in neurological problems, such as increased irritability, shyness, tremors, vision and/or hearing loss, mental retardation, and memory loss. It is also harmful to the kidneys. It is especially hazardous for the fetus, infants, and young children. As many as 4.9 million women of childbearing age in the United Statesmay have unsafe levels of mercury, and as many as 60,000 newborns per year are at risk due to dietary exposure. Fish can be eaten regularly, but some care should be taken in terms of the quantity, frequency, and type of fish eaten. Some fish accumulate much higher levels of mercury than others. For instance, studies show that swordfish tend to have much higher levels of mercury than salmon, so it might be wise to eat swordfish only occasionally (see also Chapter 26).

may eventually be fixed as limestone by deposition of the skeletons of some marine invertebrates in oceans. The atmosphere currently contains about 750 billion metric tons (BMT) of carbon in the form of CO2, and a 3-BMT excess enters the atmosphere each year. Research indicates that this excess gives rise to a mean annual increase of about 1.5 μL L–1 in the global concentration of CO2. In addition to CO2, the other main greenhouse gases are CH4, N2O, chlorofluorocarbons(CFCs), and O3. Water vapor is also an important, but variable, greenhouse gas. All of these gases are, as the term “greenhouse” implies, efficient absorbers of longwave radiation. This absorption helps maintain a relatively warm climate on earth. However, because greenhouse gas concentrations continue to increase, the earth’s climate is undergoing rapid alteration and global temperatures are rising. Therefore much scientific research is currently being directed toward improving our understanding of the atmospheric budgets of these trace gases and their role in the greenhouse effect. Atmospheric CH4 concentrations have also steadily increased at a rate of about 1% per year in recent decades.

17

301

This rapid increase is commonly attributed to increased worldwide rice and livestock production. Increased mining of natural gas resources for energy production may also be an important factor. Synthetic CFCs are also significant contributors to the greenhouse effect. Chlorofluorocarbons are used in refrigerators, air conditioners, foam insulation, and industrial processes. In addition to being very efficient longwave absorbers, CFCs are also involved in depleting stratospheric O3. Fortunately, because of concerted international effort resulting in the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, CFC emissions to the atmosphere have decreased substantially in recent years (see Chapter 32 for more details on the Montreal Protocol). Nitrous oxide (N2O) is an especially good absorber of longwave radiation; 1 molecule of N2O is equivalent to about 200 CO2 molecules in terms of its ability to absorb longwave radiation. Currently, atmospheric N2O accounts for only about 5% of the greenhouse effect, but this percentage is expected to increase in coming years. A 25% N2O increase in atmospheric concentration may, according to numerical model predictions, increase global mean temperature by about 0.1 K. Worse, N2O has a very long atmospheric lifetime, estimated to be about 150 years, which is far longer than the atmospheric lifetime of any other nitrogen oxide. Thus the current buildup of N2O could affect the earth’s climate well beyond the 21st century.

17.2.4.2 Stratospheric Pollution Stratospheric O3 depletion is another global environmental concern related to pollution. Concern about O3 first emerged in the early 1970s, when modeling studies indicated that a proposed fleet of supersonic transport (SST) aircraft could emit enough NOx to damage the O3 layer. The results from the modeling studies helped put an end to plans to build the fleet, but such considerations remain major factors in plans for aircraft development.In the mid-1970s, concern shifted to the possible O3-depleting effects of manufactured CFCs used as refrigerants, propellants, cleaning compounds, and foam insulation. Intensive study of the effects of CFCs on stratospheric O3 led to a 1979 U.S. ban on the use of CFC propellants in aerosol spray cans. Stratospheric O3 absorbs UV light, decreasing the amount of UV striking living organisms on the earth’s surface. Satellite and ground-based measurements have shown that there is a temporary decrease in O3 concentrations (50%–75% of total) over Antarctica each year. This has come to be known as the ozone hole, which is defined as the geographic area above the Antarctic where the total ozone is less than 220 Dobson units between 1 October and 30 November (Fig. 17.9). Fig. 17.10 shows

302

PART

II Environmental Pollution

FIG. 17.9 Southern Hemisphere map of total ozone for September 2004. The ozone hole, with total ozone lower than 220 Dobson Units, is shown in purple and magenta. (Image: U.S. National Oceanic and Atmospheric Administration.)

FIG. 17.10 Year-to-year variations in the average size of the Antarctic ozone hole between 1 October and 30 November. The size of the ozone hole appears to be decreasing since the phaseout of CFCs. (Data: U.S. National Oceanic and Atmospheric Administration.)

the average area of the Antarctic ozone hole in recent years. Stratospheric O3 depletion has engendered serious concerns about the causes and possible ecological and human health consequences if this trend continues. In humans, increased UV would probably increase the incidence of skin cancer, including melanoma. Other organisms are also vulnerable to UV; phytoplankton, for example, has declined by 6%–12% in areas near Antarctica. The decline in this one-celled organism is thought to be due to increased amounts of UV that are reaching surface waters. The chemical pathways leading to the formation of stratospheric O3 start with the photodissociation of molecular oxygen (O2) by solar UV radiation (photons of energy hv, where h is Planck’s constant and v is the frequency). The UV photon splits O2 into two oxygen atoms (O), each of which recombines with undissociated O2 (in the presence of another chemical species, M) to form two O3 molecules. These two reactions, which result in a net formation of O3 are given as follows:

Atmospheric Pollution Chapter

O2 + hv

!O+O

2O2 + 2O + M ! 2O3 + M Net Reaction : 3O2 + hv ! 2O3

(17.2)

The two O3 molecules quickly convert back to molecular oxygen via (17.3)

The process of production and loss of the O3 molecules by photodissociation is very important because, overall, it helps prevent harmful UV from reaching the earth’s surface. These production/destruction schemes indicate that the chemistry of stratospheric O3 would be straightforward if there were no other reactive chemical species in the stratosphere. However, other chemicals, such as CFCs and NOx, are present and play an important destructive role. This is indicated by the following general catalytic cycle: X + O3 ! XO + O2 O3 + hv ! O + O2 XO + O ! X + O2 2O3 + hv ! 3O3

Net Reaction :

(17.4)

where X and XO represent the compounds or free radicals that catalyze the destruction of O3 molecules. Mainly NOx, water vapor, and CFCs, these species are summarized in Table 17.1. Of these catalysts, CFCs are entirely anthropogenic, whereas nitrogen oxides come from both natural and synthetic sources. Stratospheric water vapor comes mainly from natural processes. In addition to the three main catalysts, other chemicals may play a role in controlling stratospheric O3 levels. For example, recent evidence indicates that methyl bromide, which is used as a soil fumigant, may reach the stratosphere, where it can undergo a catalytic reaction sequence with O3 similar to those of the three main chemical species. The catalytic reactions do not destroy all the O3 present in the stratosphere. The reason they donot is that reactions also occur between the catalysts, and these reactions result in chemicals that do not deplete O3. Some of the chemicals

TABLE 17.1 Chemical Species that are Believed to Catalyze the Destruction of O3 Molecules in the Atmosphere Cycle

X

XO

NOx

NO

NO2

Water

HO

HO2

CFC

Cl

ClO

303

eventually return to the earth’s surface (e.g., HNO3 in rain). Further aspects of each of these catalytic cycles are described in Graedel and Crutzen (1993) and are briefly discussed in the following paragraphs. NOx/O3Destruction Cycle

O3 + hv ! O + O2 O3 + O ! 2O Net Reaction : 2O3 + hv ! 3O3

17

The nitrogen oxides in the stratosphere come mainly from photodissociation of nitrous oxide, which originates mostly from microbial processes at the earth’s surface. Nitrous oxide is also a major greenhouse gas. It is produced mainly within moist soils by microbial denitrification of nitrate fertilizer, but it can also be biologically produced in oceans. Since the 1970s there has been concern that increased agricultural use of nitrogen fertilizers could increase the amount of nitrous oxide reaching the stratosphere, ultimately depleting O3. Measurements of atmospheric N2O indicate that its concentration is increasing by about 0.25% per year. A 25% increase in N2O by the late 21st century could reduce total stratospheric O3 by 3%–4%, which could increase the incidence of skin cancer by 2%–10%. Nitrous oxide is not known to be lost within the troposphere; however, it is converted to NO in the stratosphere mainly by the following reactions: N2 O + hv ! N2 Oð1 DÞ N2 O + Oð1 DÞ ! 2NO

(17.5)

The two NO molecules formed initiate the O3 destruction reactions described previously. Note that O(1D) in Eq.(17.5) denotes atomic oxygen in an electronically excited state. H2O/O3Destruction Cycle The stratosphere is generally very dry. However, enough water vapor is present to react with electronically excited atomic oxygen to produce the free radical HO via H2 O + O


1  D ! 2HO

(17.6)

The catalytic water cycle has less influence upon O3 concentrations than do the other reaction cycles, but it can be significant when sufficient water vapor is present. CFC/O3Destruction Cycle Chlorofluorocarbons (e.g., CFCl3 and CF2Cl2) are relatively stable in the troposphere, but once in the stratosphere, they are photodissociated by UV. This photodissociation produces the catalysts Cl and ClO, both of which deplete O3. There is evidence that links the O3 depletion in the Antarctic region to CFCs and other pollutants that carry chlorine and bromine into the stratosphere. Chlorine monoxide (ClO) has been identified as the chief cause of O3

304

PART

II Environmental Pollution

depletion in polar regions. Weather patterns and volcanic eruptions may also play a part. Chlorofluorocarbons are also implicated in possible global climate change. Because many are extremely efficient absorbers of longwave radiation, they contribute to the earth’s greenhouse effect.

17.3

WEATHER AND POLLUTANTS

What happens to pollutants in the atmosphere? The answer depends on several factors. Pollutants are transported by wind and turbulence, and they may undergo chemical transformations before being deposited on the earth’s surface. Thus weather conditions strongly affect the fate of air pollutants.

17.3.1

Stability and Inversions

The stability of boundary-layer air (see Chapter 4) largely determines how quickly pollutants are moved upward from their ground sources. Stability is primarily a function of the vertical air temperature gradient relative to the adiabatic lapse rate. Strong instability associated with buoyancy causes efficient air mixing and pollution dispersal over a large mixing depth of the boundary layer (from 100 to 1000 m). Good mixing often occurs on warm days when the ground is heated by sunlight. In contrast, pollution is poorly dispersed on days or nights when the atmosphere is stable. At those times, turbulent movement of pollution upward is slow or nearly nonexistent. We know from Chapter 4 that temperature inversions influence atmospheric stability; thus they play an important role in determining the concentrations of air pollutants. The effects of inversions are intensified by limited air drainage out of enclosed valleys, as is the case in Los Angeles (Fig. 17.7) and Mexico City. Various processes may generate inversions, including surface cooling caused by loss of infrared radiation or by evaporation, atmospheric subsidence, and topographic effects. Ground-surface cooling is caused mainly by infrared radiation emission from the surface to the sky. It generally occurs during clear, calm nights, with inversion heights extending about 100 m above the ground. Such inversions commonly occur throughout the western United States during fall, winter, and spring, when the air is relatively dry and skies are clear. Such conditions readily permit cooling by longwave loss of energy from the ground. Tucson, Arizona, for example, often experiences radiation inversions in the cooler months, so that wintertime pollution problems are exacerbated in the area. Cooling of the ground by evaporation of water from soil and plants may also establish inversions. Evaporative cooling can occur during the day or night, particularly over irrigated fields. This type of inversion may be important in relation to certain agricultural activities, such as the aerial application of pesticides

over large irrigated fields. The depth of an evaporatively cooled inversion layer is usually just a few meters. Warming of the atmosphere by subsidence causes inversions over regions that have semipermanent high pressure (anticyclonic flow), such as the southwestern United States. As air subsides (sinks), it encounters higher pressure and thus is warmed. Within regions of high pressure, an inversion height may be several hundred meters above ground; thus the air may be very stable over a large depth of the atmosphere. Because subsidence inversions may last from several days to weeks, the result is highly polluted conditions at ground level. For example, subsidence inversions are a major factor in reducing air quality in the Los Angeles basin. Inversions associated with topography result from adiabatic warming of air as it flows downslope over mountainous terrain. These inversions may exacerbate air pollution problems in populated, mountainous areas such as Denver, Colorado, or Salt Lake City, Utah.

17.3.2 Wind and Turbulence in Relation to Air Pollution Wind affects turbulence near the ground, thus affecting the dispersion of pollutants released into the air. Turbulence (largely fine scale vertical and horizontal motion of air) is generated in part by air flow over rough ground. The greater the wind speed, the greater the turbulence, and hence the greater the dispersion of pollutants that are near the ground (Fig. 17.11). We can visualize the dispersion of pollutants in air by looking at the familiar cloud or “plume” of pollution emitted continuously by a smokestack (Fig. 17.12). As the plume contents are carried away from the stack by the wind, the size of the plume increases owing to dispersion. Because of dispersion, the pollutant concentration within the plume decreases with increasing distance from the source. Dispersion of pollution downwind from a smokestack is affected by the roughness of the ground surface. Because of friction between the atmosphere and the ground, wind speed is slowed markedly near the ground. If the surface is relatively rough, as it is when trees and buildings are present, the air flow tends to be turbulent and the increase of wind speed with increasing height is relatively small. Greater surface roughness increases turbulence, which helps disperse pollutants. Air flow over a smooth surface, such as a large mowed lawn, tends to be less turbulent and the decrease in wind speed near the ground is relatively small (see Fig. 17.11). The cone-shaped plume in Fig. 17.12 illustrates the pattern of pollutant dispersal downwind of a point source. Several factors affect the plume, including the effective height (H) of emission, which is a measure of how high the pollutants are emitted into the atmosphere directly

Atmospheric Pollution Chapter

17

305

Totally unaffected by surface

Moderately unaffected by surface

Free convection

Mixing depth Thermal updrafts

Completely controlled by surface Forced convection

Mountain City

Forest

FIG. 17.11 Diagrammatic representation of air flow, mixing, and relative velocity over varying terrain as affected by height. (From Pollution Science © 1996, Academic Press, San Diego, CA.)

Usually, turbulence helps mix plume contents uniformly in such a way that the concentration follows a Gaussian distribution about the plume’s central axis. Mathematically, pollutant concentration χ (x,y,z,H) (kg m–3) at any point in the plume is described by χ ðx, y, z, HÞ ¼

Q 2πσ y σ z u

y2 exp  2 2σ y " FIG. 17.12 Plume pattern (coning) resulting from continuous stack emission into a near-neutral stable boundary layer under moderate winds. (From Pollution Science © 1996, Academic Press, San Diego, CA.)

above the source. The height is dependent upon source characteristics and atmospheric conditions. Generally, a tall smokestack produces relatively low ground-level pollutant concentrations, because turbulence tends to dilute the pollution before reaching the ground. Driven by buoyancy, fast-moving pollutants are initially transported high up into the atmospheric boundary layer because they are warmer than the surrounding air. But as the pollutants cool and merge with the ambient air, the plume begins to move sideways with the wind. Then turbulence caused by the air flow over the surface and by possible instability governs the diffusion of the plume contents.

ðz  H Þ2 exp  2σ 2z

!

!

ðz  H Þ2 + exp  2σ 2z

(17.7) !#

where: Q is the rate of emission of pollution from the source (kg s–1) σ y and σ z are the horizontal and vertical standard deviations of the pollutant concentration distributions in the y and z directions u is the mean horizontal wind speed within the plume (m s–1). This model, which is applicable to continuous sources of gases and particulates less than about 10 μm in diameter (larger particles quickly settle to the ground), can be used to model plume concentrations over horizontal distances of 102–104 m. With this Gaussian plume model, it is

306

PART

II Environmental Pollution

assumed that no deposition of plume contents to the ground surface takes place. In fact, it is assumed that plume contents are “reflected” from the ground back to the air. The values of σ in the equation are estimated from any one of several empirical formulas that relate σ to downwind distance (x) and stability conditions. These formulas include the following equations, which were developed by the Brookhaven National Laboratory (BNL). σ y ¼ axb and σ z ¼ cxd

(17.8)

where a, b, c, and d are parameters dependent upon stability. (See Hanna et al., 1982 for a summary discussion of BNL equations as well as other approaches.) At ground level, z ¼ 0, and along the plume centerline, y ¼ 0. Thus, from Eq. (17.7), the concentration can be calculated by   Q H2 exp  2 χ ðx, HÞ ¼ 2σ z πσ y σ z u Q H2 exp 
2 ¼ b d πax cx u 2 cd d

!

(17.9)

One type of plume (shown in Fig. 17.12) typically occurs under windy conditions with stability conditions at or near neutral. Within such a plume, mixing occurs mainly by frictionally generated turbulence, and pollutant diffusion is nearly equal in all directions (i.e., the σ values are nearly equal and the plume spreads out in the familiar cone pattern, known as coning). Coning can occur day or night, and is often seen during cloudy and windy conditions. Depending upon effective source height and atmospheric conditions, the plume may reach the ground close to the source. Using Eqs. (17.8), (17.9), we can estimate the ground level (z ¼ 0) concentration of the plume composed of a pollutant, say, SO2, emitted into the atmosphere at a known effective height. Suppose we have the following: Q ¼ 0.5 kg s–1, H ¼ 25 m;u ¼ 2 m s–1; near neutral stability, and BNL parameters a ¼ 0.32, b ¼ 0.78, c ¼ 0.22, and d ¼ 0.78. Then the ground-level SO2 concentration along the plume centerline at an arbitrary distance of x ¼ 500 m from the source will be 4.7  10–5 kg m–3 (or 47 mg m–3). Plumes may change because of changes in the wind velocity and boundary-layer stability. When the atmospheric boundary layer is strongly stable, such as during radiation inversions at night or during subsidence inversions, a fanning pattern may be evident in the plume, as illustrated in Fig. 17.13A. Under these conditions, there is almost no vertical motion and the BNL parameters are a ¼ 0.31, b ¼ 0.71, c ¼ 0.06, and d ¼ 0.71. Lack of vertical motion thus effectively forces the plume into a relatively narrow layer, while changes in wind direction may spread the plume out laterally, resulting in a V- or fan pattern; hence, the term. A constant wind direction, however, forces the plume into a tightly closed fan pattern, which follows a relatively straight and narrow path. Over flat terrain the plume in Fig. 17.13A

FIG. 17.13 Effect of atmospheric stability upon resultant stack plume pattern during (A) inversion (fanning pattern), (B) dissipation of inversion near ground (fumigation pattern), (C) lapse conditions (looping pattern), and (D) lofting pattern.

may be unchanged for very long distances. If there is no vertical air movement, ground-level concentrations downwind of a tall smokestack can be nearly zero. However, if the source is close to the ground (i.e., H is small), or if changes in topography cause the plume to intercept the ground, the ground-level concentrations can be very large.

Atmospheric Pollution Chapter

By midmorning, surface heating by solar radiation typically begins to break down the inversion developed during the previous night, as illustrated in Fig. 17.13B. Unstable conditions develop near the ground, resulting in vertical mixing of the air. With moderately unstable conditions, a ¼ 0.36, b ¼ 0.86, c ¼ 0.22, and d ¼ 0.86. In this situation, pollution is transported downward toward ground level. Stable conditions above, however, limit dispersion of pollutants upward. Thus the remaining inversion effectively puts a “lid” over the ground-level pollution. This situation is known as fumigation and generally lasts for periods of an hour or less. Fumigation is highly conducive to enhanced ground-level pollutant concentrations. By early afternoon, lapse conditions (i.e., negative vertical temperature gradient) generally become fully established within the boundary layer due to strong surface heating by the sun. During much of the afternoon, air motion mainly exhibits the large turbulent eddies associated with buoyancy. These eddies are generally larger than the plume diameter and thus transport the plume upward and downward in a sinusoidal path or looping pattern, as illustrated in Fig. 17.13C. The loops are carried with the overall wind pattern and generally increase in size with increasing distance downwind from the source. The motion may bring the plume contents to ground level quite close to the source. Because of turbulence, however, the plume eventually becomes dispersed at relatively large distances. By early evening, a radiation inversion often rebuilds from ground level upward. Stable conditions near the ground inhibit transport of plume contents downward, but unstable air aloft (above the inversion height) allows dispersal upward. This upward transport, known as lofting, is highly favorable for dispersing pollutants, as shown in Fig. 17.13D. Lofting is only effective when the source is above the inversion height. Plume contents emitted below the inversion height are essentially trapped in a fan-type plume configuration. Topography downwind from pollution sources affects air quality, especially in mountainous areas. For example, air drainage into relatively enclosed valleys during winter and/ or inversion conditions can cause accumulation of pollutants within the valleys. Thus urban areas in valleys with restricted air flow are particularly prone to high pollution levels. In addition, in coastal areas, air flow from the ocean (sea breezes) can be blocked or channeled by mountain ranges. This situation is common in the Los Angeles basin, which is surrounded by mountains that restrict air flow from the Pacific Ocean. Thus dispersal of pollutants from sources in the basin is inhibited.

17

307

of these chemical reactions are poorly understood, they are an important factor affecting the fates of many air pollutants. Most pollutants, such as CO, remain in the atmosphere for relatively brief periods, lasting only a few days or weeks. Thus, if emissions were completely curtailed, the lower atmosphere would quickly lose nearly all of its pollutants. However, some pollutants—volcanic ash and sulfur-containing aerosols, for example—emitted high into the stratosphere can remain there for months before settling back to the surface. These long-lasting upper-atmospheric pollutants can alter the earth’s climate, as evidenced by lower air temperatures resulting from volcanic eruptions (see Chapter 4). In addition, synthetic chlorofluorocarbon (CFC) compounds can remain in the atmosphere for many years before they break down. Pollution can be removed from the atmosphere by gravitational settling, dry deposition, condensation, and wet deposition. Gravitational settling. Gravitational settling removes most particles whose diameters are greater than about 10 μm. Particles less than 10 μm in diameter are often small enough to stay suspended in the atmosphere for long periods. Particles greater than about 10 μm in diameter quickly settle out. Dry deposition. Dry deposition is a mass-transfer process that results in adsorption of gaseous pollutants by plants and soil. Dry deposition to plants is dependent upon uptake of the pollution through stomatal openings in plant leaves and upon turbulent transport in the air. Dry deposition to bare soil involves not only turbulent transport of pollutants in air above the soil, but also soil microorganisms that take up such pollutants such as CO. Condensation. Volatile organic compounds can condense on cold surfaces during winter in temperate and polar regions. The process of evaporation, transport, and condensation of toxic compounds, such as dioxins and the pesticide, dichlorodiphenyltrichloroethane (DDT), may be responsible for causing high levels of toxic organic pollutants in the Arctic (see Chapter 25). Wet deposition. Rain is very effective at removing gases and small particulates. Raindrops increase in size as they fall toward the ground, and thus they increasingly capture more pollutants. Raindrops, in effect, “sweep up” pollution as it falls through the air. The ability of the rain to remove pollutants depends upon the rainfall intensity, the size and electrical properties of the drops, and the solubility of the polluting species.

17.3.2.1 Pollutant Transformation and Removal

17.4 POLLUTION TRENDS IN THE UNITED STATES

As pollutants move with the wind, chemical reactions often occur between the pollutants and other atmospheric chemical species. Although the pathways and rates of many

Emissions of nearly all types of primary air pollutants have generally declined or held steady in the United States since about 1970 (Fig. 17.14). This decline is mostly attributable

308

PART

II Environmental Pollution

gasoline is also used because leaded fuels deactivate catalytic converters. It is generally recognized that reducing air pollution through control of emissions at the source is the best approach, which is the goal of the EPA and other regulatory agencies. Total control of pollutant emissions is certainly not feasible for various economic and technological reasons, but efforts at reducing emissions are helping to improve air quality in most locations. There are various physical and chemical precipitators/concentrators/burners that can be used to control emissions. Consult an environmental engineering handbook for more details.

QUESTIONS AND PROBLEMS

FIG. 17.14 Air pollution emissions have generally decreased since the 1970s with the exception of PM10. PM10 emissions rose sharply in 1985, largely due to the adoption of reporting PM10 emissions in the miscellaneous category, which includes roads, construction, and agriculture. (A) CO and lead, (B) Other pollutants. (Data: U.S. Environmental Protection Agency.)

to general compliance with the federal air quality regulations set forth in response to the U.S. Clean Air Act of 1970. Although air quality is improving overall, many specific urban areas fail to meet the air quality standards set for some pollutants. Poor air quality is estimated to affect the lives of about 100 million people in the United States alone. Despite the fact that transportation continues to be a major source of pollution in the United States, the proportion of its contribution is diminishing. While the number of cars is increasing in most urban areas, fuel efficiencies have increased and pollutant emissions per vehicle are declining owing to improvements in technology such as catalytic converters and other pollution control devices. Evidence of improved air quality is shown by the marked decline of atmospheric lead (Pb) concentrations since 1970. Atmospheric lead comes mainly from the burning of lead-containing gasoline in cars and trucks. Thus the introduction of unleaded gasoline was a significant factor in this decline. Now required for cars in the United States because of environmental health concerns, unleaded

1. Describe how surface air temperature inversions form. Why are airtemperature inversions important relative to air pollution in urban areas? 2. What factors affect atmospheric stability? Explain. 3. Based upon the Brookhaven National Laboratory Eq. (17.8), how do numerical values of σ z values compare at x ¼ 100 m for pollution plumes during stable and unstable atmospheric conditions? How do the σ y values compare? During which condition (stable or unstable) would you expect the plume to intercept the ground closer to the source? Explain. 4. Describe the processes that remove air pollution. 5. What is the difference between EPA-designated primary and secondary air pollution? Give an example of each type of pollutant. 6. What is photochemical smog? Explain how it is formed. 7. Explain how O3 in the stratosphere is beneficial, whereas O3 in the troposphere is harmful. 8. Explain how anthropogenic chlorofluorocarbons (CFCs) destroy stratospheric O3.

REFERENCES Graedel, T.E., Crutzen, P.J., 1993. Atmospheric Change: An Earth System Perspective. W.H. Freeman, New York. Hanna, S.R., Briggs, G.A., Hosker Jr., R.P., 1982. Handbook on Atmospheric Diffusion. U.S. Department of Energy, Washington, DC.

FURTHER READING Ahrens, C.D., 2003. Meteorology Today: An Introduction to Weather, Climate and the Environment, seventh ed. Brooks/Cole-Thomson Learning Inc., Pacific Grove, CA. Albritton, D.L., Monastersky, R., Eddy, J.A., Hall, J.M., Shea, E., 1992. Our Ozone Shield: Reports to the Nation on Our Changing Planet. Fall 1992. University Cooperation for Atmospheric Research, Office for Interdisciplinary Studies, Boulder, CO. Dickinson, R., Monastersky, R., Eddy, J., Bryan, K., Matthews, S., 1991. The Climate System: Reports to the Nation on Our Changing Planet.

Atmospheric Pollution Chapter

Winter 1991. University Cooperation for Atmospheric Research, Office for Interdisciplinary Studies, Boulder, CO. EPA. (2005) Asthma research results highlights. EPA 600/R-04/161. Available from:http://www.epa.gov/ord/articles/2005/asthma_fact_sheet.htm. Mitchell, J.F.B., 1989. The “greenhouse” effect and climate change. Rev. Geophys. 27, 115–139. Oke, T.R., 1987. Boundary Layer Climates. Routledge, New York.

17

309

Ozone Transport Assessment Group, 1997. Telling the OTAGozone story with data. In: Final Report, Vol. I: Executive Summary. OTAG Air Quality Analysis Workgroup. Co-chairs, D. Guinnup and B. Collom. Available from: http://capita.wustl.edu/otag/reports/ aqafinvol_I/animations/v1_exsumanimb.html. Schlesinger, W.H., 1991. Biogeochemistry: An Analysis of Global Change. Academic Press, San Diego, CA.